Exhaust emissions from motor vehicles are a significant source of air pollution and are major contributors to the photochemical smog and ozone events which have been correlated to significant adverse impacts on health (M. V. Twigg, Applied Catalysis B, vol. 70, (2007), p 2-25). Hence over the last thirty years increasingly stringent legislative limits have been introduced to regulate the emissions from both petrol/gasoline and diesel internal combustion engines e.g. Euro 5 or Euro 6 (Regulation (EC) No 715/2007 of the European Parliament and of the Council, 20 Jun. 2007, Official Journal of the European Union L 171/1, see also Twigg, Applied Catalysis B, vol. 70 p 2-25 and R. M. Heck, R. J. Farrauto Applied Catalysis A vol. 221, (2001), p 443-457 and references therein). The most significant gaseous vehicular emissions comprise pollutants such as carbon monoxide (CO), oxides of nitrogen (NO and NO2 collectively NOx), and unburnt hydrocarbons (HC). To achieve the legally required remediation goals, exhaust after-treatment technologies have been developed for both gasoline and diesel engines. These technologies include, but are not limited to, engine control methodologies/modification, alternate combustion cycles and the use of after-treatment systems e.g. catalytic control devices which eliminate exhaust pollutants by promoting chemical changes to convert unwanted compounds into more benign species. In the case of diesel/compression ignition engines the latter devices include the Diesel Oxidation Catalyst (DOC), Diesel NOx Trap/NOx Storage Catalyst (DNT/NSC) and Selective Catalytic Reduction catalyst (SCR) to address emissions of CO, HC (DOC) and NOx and the use of the Catalysed Diesel Particulate Filter (CDPF) for the removal and combustion of entrained solids, also known as particulate matter or soot.
Of the aforementioned catalytic systems for diesel emission control the DOC is both the most widely studied and implemented technology (for examples see U.S. Pat. No. 5,371,056, U.S. Pat. No. 5,462,907, U.S. Pat. No. 6,153,160, U.S. Pat. No. 6,274,107, J. A. A. van den Tillaart, J. Leyrer, S. Eckhoff and E. S. Lox in Applied Catalysis B Vol 10, 1-3, p 53-68). Current ‘conventional’ DOCs comprise a refractory oxide support e.g. Alumina, a hydrocarbon storage/release component to enhance low temperature performance, typically a Zeolite (Applied Catalysis B, vol. 70, (2007), p 2-25, Applied Catalysis A vol. 221, (2001), p 443-457) and an active Precious Group Metal (PGM) or metals, initially Pt or more recently the combination of Pt/Pd as the primary catalytic materials e.g. see U. Neuhausen, K. V. Klementiev, F.-W. Schütze, G. Miehe, H. Fuess and E. S. Lox in Applied Catalysis B: Environmental, Vol 60, 3-4, (2005), p 191-199 and references therein. The choice of these metals is based upon their ability to offer the highest turnover (number of reactions per second) with respect to the oxidation of CO and Hydrocarbon to CO2 and water at low temperatures and low concentrations of active component within the DOC formulation.
The requirement of the DOC with respect to direct control of gaseous emission has been augmented over time to meet specific new challenges arising from each generation of legalisation, e.g. the ability to efficiency combust post-injected HCs to generate the thermal ‘bloom’ required to initiate DPF regeneration or more recently the ability to oxidise NO to NO2 in order to facilitate low temperature NH3—SCR chemistry. Moreover, this multi-functionality must be incorporated without detriment to the primary role of the DOC for effective emission control i.e. the DOC must posses a Low Temperature ‘light off’. Thus in addition to such multi-functionality the DOC must provide operation at low temperatures to minimise ‘cold-start’ emissions. This requirement is especially critical given the increasingly lower temperature window of operation of the current and next generation diesel engines, which present increasing CO and HC emissions arising from the increased use of exhaust gas recirculation or advanced combustion cycles employed to decrease engine out NOx levels (patent WO/2005/031132, Method and Apparatus for Providing for High EGR Gaseous-Fuelled Direct Injection Internal Combustion Engine). This challenge is rendered yet more difficult due to the intrinsic kinetics of CO oxidation, wherein higher concentrations of CO are self-inhibitory to the rate of oxidation (A. Bourane and D. Bianchi J. Catalysis 222 (2004) 499-510 and references therein). A further and final requirement is that the DOC must maintain this high level of activity after exposure to transient high temperatures in the presence of steam as occurs for a close-coupled catalyst or during the active regeneration strategy required for the DPF, as a result of the exotherm generated in the DOC by the combustion of post-injected hydrocarbons.
In order to fulfil the aforementioned targets, and also comply with end-of-life performance targets, it has therefore been necessary to increase the PGM content of conventional DOCs. This in turn has increased demand for Platinum (Pt) and Palladium (Pd) resulting in further price pressure for these PGMs and also for vehicle manufacturers. Hence what are required to alleviate these issues are alternative, more cost effective, base metal catalysts to replace or augment the conventional PGM function in the DOC. These base metal catalysts must offer competitive, hydrothermally durable and poison resistant activity under the diverse conditions of the diesel exhaust environment.